Flexible Sensors based on Radiation Induced
Diffusion of Ag in Chalcogenide Glass
P. Dandamudi, Student Member, IEEE, A. Mahmud, Y. Gonzalez-Velo, Member, IEEE, M. N.
Kozicki, Member, IEEE, H. J. Barnaby, Senior Member, IEEE, B. Roos, T. L. Alford, M.
Ailavajhala, Student Member, IEEE, M. Mitkova, Member, IEEE, and K. E. Holbert, Senior Member,
IEEE

Abstract— In this study, previous work on chalcogenide glass
(ChG) based radiation sensors is extended to include the effects
of mechanical strain and temperature stress on sensors formed
on a flexible polymer substrate. We demonstrate the feasibility of
producing inexpensive flexible radiation sensors, which utilize
radiation-induced migration of Ag+ ions in germanium selenide
(Ge20Se80) films to produce a decrease in resistance of several
orders of magnitude between surface electrodes. This change in
resistance can be related to total ionizing dose to give an
instantaneous readout of radiation exposure. The ChG films are
inherently flexible and this, along with an extremely simple
device fabrication process at or near room temperature, allows
inexpensive sensor structures to be fabricated on lightweight
pliable polymeric substrates such as polyethylene napthalate
(PEN). Test samples were irradiated with ionizing radiation (UV
light and 60Cobalt gamma rays). Irradiated samples were
subjected to both tensile and compressive stress, and elevated
operating temperatures. Stress and exposure to increased
ambient temperature had little effect on device resistance.
Analysis of the experimental data is supported by the results of
COMSOL simulations that model radiation-induced lateral Ag
diffusion in ChG.
Index Terms— Polyethylene napthalate (PEN), chalcogenide
glass, UV, gamma rays, flexible radiation sensor, dosimetry
I. INTRODUCTION
R
ADIATION sensors have a number of applications, from
survey monitors which are used to supervise the
generated radioactive wastes at nuclear power plants to
personal dosimeters which measure the radiation dose
accumulated in individuals. Most of the existing personal
dosimeters (e.g. thermoluminescent dosimeters) are costly to
process and suffer from poor signal retention [1], [7] - [9], and
solid-state crystalline detectors (Ge, SiGe, SiLi) require
cryogenic temperatures to operate precisely and accurately.
This work was funded in part by the Defense Threat Reduction Agency
under grant no. HDTRA1-11-1-0055.
Pradeep Dandamudi, A. Mahmud, Y. Gonzalez-Velo, M. N. Kozicki, H. J.
Barnaby, and K. E. Holbert are with the School of Electrical, Computer and
Energy Engineering, Arizona State University, Tempe, AZ 85287-5706 USA.
(e-mail: amahmud1@asu.edu).
B. Roos and T. L. Alford are with School for Engineering of Matter,
Transport and Energy, Arizona State University, Tempe, AZ 85287-5706
USA.
M. Ailavajhala and M. Mitkova are with the Department of Electrical and
Computer Engineering, Boise State University, Boise, ID 83725 USA.
Additionally, existing detectors are often stand-alone
devices/systems, requiring complex workarounds to
incorporate their detection capabilities into external circuitry
[7] – [9]. We have demonstrated that thin film chalcogenide
glass (ChG) based radiation sensors have the potential to
perform similar functions to their conventional counterparts
while being far simpler in form and function, and potentially
cheaper to produce and operate [1].
II. SENSOR OVERVIEW
Chalcogenide glasses (ChG) are a recognized family of
amorphous glass containing chalcogen atoms (sulfur (S),
selenium (Se), Tellurium (Te)) in conjunction with more
electropositive either group IV elements (i.e., Ge, Si) or group
V elements (i.e., Sb, As) [5]. The properties of ChG are
dependent on their composition. By changing the composition,
these properties can be modified, which offers the opportunity
to use them for various electronic, photonic and optoelectronic
applications. ChG thin films are innately supple and have been
extensively investigated as materials in flexible optical fibers
[10] – [12]. The effect of Ag photodoping in thin ChG films
has led to development of high-resolution inorganic
photoresists for optical lithography in semiconductor
manufacturing [6]. The addition of some metals into ChG can
result in a significant increase in the electronic conductivity of
the material [13] – [14]. In case of cation (M+) migration,
nano-scale group I elements (i.e., Ag or Cu) are dispersed into
ChG to form binary and ternary solid electrolytes. These
chalcogenide glass based solid electrolytes enable the essential
resistance switching mechanism for Programmable
Metallization Cells (PMC), a technology platform for
Conductive Bridging Random Access Memory (CBRAM) [6].
The weak bonding that give ChG’s their flexibility is also the
source of their sensitivity to ionizing electromagnetic
radiation, allowing some metals to be dissolved into their
backbone structure when exposed [15]. The nature of the ChG
allows for the generation of charges and defects as well as
structural modifications upon exposure to high energy ionizing
radiation. These radiation-induced processes facilitate the
incorporation of Ag into ChG, thereby changing film
conductivity which may be measured electrically [16] – [17].
The primary mechanism for the flexible sensors is
radiation-induced diffusion of metal atoms, typically Ag, into
the ChG (GexSe1-x in our studies), which results in change in
the resistivity of the glass. The more Ag dissolved in the ChG,
the lower the resistance of the resulting Ag-Ge-Se ternary. In
addition, sensor operation does not require an applied bias
during exposure and can be related to the dose, via calibration
and modeling, to provide an instantaneous readout at very low
voltage. Fig. 1 shows the overview of the ChG sensor. The
device is compact in size, mass-manufacturable, and can be
easily incorporated in the back end of a standard integrated
circuit process flow, making it a good candidate for the next
generation of portable, electronic, field dosimeters.
the ChG at a rate of 0.1 nm/s using the same tool but this time
a shadow mask is used to form square arrays of 50 nm thick, 2
mm diameter circular electrodes with 1 mm spacing. The
sensor cross-section is shown in Fig. 3.
Fig. 1. ChG sensor overview: When the ChG sensors are exposed to ionizing
radiation, the deposited energy causes Ag dissolution in ChG and the resulting
resistivity change can be detected by the electrical resistance at the output.
Our previous work concentrated on ChG radiation sensors
fabricated on rigid substrates [1] but in this paper we show the
results of ChG-based sensors formed on a flexible polymer
substrate. Flexible sensors are potentially more useful than
rigid variants as they can be readily (and conformally)
attached to non-flat objects such as flasks, barrels, and pipes;
and they are inherently more robust as they will resist
breaking when mechanically stressed. They are also
potentially lighter than rigid versions as they can be formed on
thin plastic substrates. In addition, flexible substrates allow
roll-to-roll processing and hence fabrication costs can be very
low.
Fig. 2. Sensor fabrication process flow
III. DEVICE FABRICATION AND TEST PROTOCOL
The basic sensor fabrication method involves:
(1)
deposition of a thin ChG film, in this case Ge20Se80, on a
flexible substrate and (2) formation of soluble Ag electrodes.
The Ag electrodes supply the metal into the underlying ChG
during exposure as well as providing the electrical connection
to the layer for resistance measurement [1]. The Ge20Se80
composition was chosen as Ag diffuses rapidly in this Se-rich
material [18] – [21]. In addition and just as importantly for
this application, the low processing temperature (detailed
below) and very high elastic modulus of the Ge20Se80 based
glass makes this material an ideal choice for fabricating ChG
on flexible substrates. Low processing temperature is
necessary as most (low cost) polymer substrates cannot
withstand high temperatures (e.g., over 180°C). It should be
noted that the low hardness values, brittleness factor, hardness,
and Young’s modulus of the material are much lower than
silicate glass [22]; and as a result, ChG films demonstrate
more elastic deformation for the same amount of stress [23].
The fabrication sequence is depicted in Fig. 2. First, a 10
nm blanket Ge20Se80 film is deposited at room temperature
onto a 125 µm thick polyethylene napthalate (PEN) substrate
at a deposition rate of 0.1 nm/s in a Cressington 308 thermal
evaporator. Then, Ag is evaporated at room temperature onto
Fig. 3. Cross-sectional schematic of the sensor layout.
In order to assess device performance, the test samples were
exposed to UV light at power density of 2.67 mW/cm2 at a
wavelength of 324 nm for a 1 h exposure. This corresponds to
a total energy absorption density of 9.61 J/cm2. The use of UV
light was shown in our previous studies [1] – [4] to be a
simple and convenient substitute for ionizing radiation as both
generate charged carriers and induce the dissolution of the
metal into the ChG film. Samples, irradiated with UV doses
up to 57.6 J/cm2 (as shown in Fig. 4) and unexposed control
samples, were monitored to detect Ag incorporation optically
during the testing.
For 60Co gamma-ray exposure, the samples were placed in a
Gammacell 220 irradiator with a dose rate of 477.5
rad(Ge20Se80)/min. The samples were periodically removed
from the Gammacell to measure the change in electrical
resistance with respect to increasing dose levels. Samples were
exposed to a maximal total ionizing dose (TID) of 5.13
Mrad(Ge20Se80) and the samples were left floating (electrodes
unconnected) during the exposures.
(a)
IV. RESULTS AND DISCUSSION
(b)
Fig. 4. (a) Array of ChG sensors on a flexible copper-polyimide
substrate. (b) Unexposed and UV exposed sensor array on a flexible PEN
substrate. The devices are 10 nm thick Ge20Se80 film and the devices were
saturated after 57.6 J/cm2 of UV dose.
Resistance measurements were performed at room
temperature using semiconductor parameter analyzers (SPA,
Agilent 4155B for devices exposed to UV, and 4156C for
devices exposed to 60Co gamma-rays). The resistance between
two adjacent Ag electrodes was monitored for 100s at 10 mV
bias. The low measurement bias was necessary to minimize
redox reactions at the electrodes but also demonstrate that
these devices do not require high voltage for readout.
The metal photo-dissolution behavior observed in
chalcogenide glasses has been extensively studied in recent
years because of its potential application in producing highresolution lithography, electrochemical resistive memory
devices and optical components. Figs. 6a & 6b show the
photo-doping process at Ag/ChG interface under irradiation.
In the case of the photo-dissolution of Ag, light illumination
creates charged defects in the ChG and the photocarriers
absorbed at the Ag-ChG junction cause the diffusion of the Ag
into the glass. The presumed mechanism [25] is that the Ag
metal traps holes and the junction potential causes electrons to
move deeper into the ChG film and become trapped there. Fig.
6c shows an optical micrograph of an unexposed 10 nm thick
Ge20Se80 film with Ag electrodes formed on its surface. Prior
to exposure to gamma-rays, the device is in its high resistance
OFF state of around 1011 Ω, since the undoped ChG (no Ag in
film) acts primarily as a dielectric layer between the
electrodes.
(a)
(b)
(c)
(d)
Fig. 5. Flexible sensor arrays being bent around tubes of (a) 18 mm, (b) 7
mm diameter.
The sensor devices were bent around tubes of various radii
for 96 hrs and were later re-flattened for electrical
measurements. The bending operation is illustrated by the
photographs in Fig. 5. Resistance measurements were taken
before and after bending. The tensile stress was applied by
bending the devices outward while compressive stress was
applied with inward bending. All bending radii (R) are
converted to percent strain (ε) by
where
is the thickness of PEN substrate (125 µm),
is thickness of the sensor film (~ 10 nm) and
is the bending
curvature radius of the substrate [24]. A bending radius of 5
mm therefore corresponds to 1.25 % strain.
The effects of exposure to elevated temperatures on the
performance of the sensors were investigated to assess how
these devices would perform in high temperature
environments. Devices exposed to UV light and control
samples were heated on a hot plate to 75, 100, 125 and 150°C
for 1 hr. and resistance between adjacent electrodes was
measured following the temperature stress.
(e)
(f)
Fig. 6. (a, b) illustration of the the photo-doping process at the Ag-ChG
interface. (c-f) Optical micrographs show evolution of 60Co gamma
irradiation induced Ag lateral diffusion in a 10 nm thick Ge20Se80 device on a
flexible PEN substrate: (a) after 680krad(Ge20Se80) (b) after
1.22Mrad(Ge20Se80) (c) and (d) after 1.87Mrad(Ge20Se80).
The lateral progression of Ag (diffusion front) into the film
is observed in Figs. 6(c) as a change in contrast of the ChG in
the micrograph, but it is obvious that an undoped ChG layer
still exists between the Ag diffusion fronts (Fig. 6c). After
1.22 Mrad(Ge20Se80), as shown in Fig. 6d, the Ag has diffused
laterally but an undoped ChG layer remains between the two
electrodes. Hence, at this TID, the device remains in a high
resistance OFF-state. Fig. 6e and 6f show a further diffusion
of the Ag at TID close to 2 Mrad(Ge20Se80). There are no
longer undoped regions in the ChG layer and the resistance
between adjacent electrodes drops.
The evolution of resistance obtained after 60Co gamma
exposure is presented in Fig 7. It is important to note here that
reduction of resistance (from 1011 Ω to 109 Ω) of some of the
unexposed control samples were observed during our data
collection. This reduction occurs due to the natural dissolution
process of silver into the chalcogenide glass.
Resistance ()
0.0
10
12
10
11
10
10
10
9
10
8
10
7
10
6
10
5
10
4
10
3
Time3 (minutes) 3
4.2x10
8.4x10
1.3x10
sensor devices measured before bending (in the order of 10 –
12 kΩ). Excessive bending strain, caused by bending with
radius < 2 mm, induced cracks along the sensor electrodes and
this increased the device resistance to around 106 Ω (this could
be related to the cracks generated at the fringes of the substrate
while the substrate was cut for mechanical strain). The
unexposed samples did not show any Ag diffusion related to
bending strain and retained their OFF-state resistance (around
1012 Ω).
4
High Resistance State
OFF state
Sample 1
Sample 2
Sample 3
Sample 4
Sample 5
Control
Fig. 8. ON-state resistance as a function of applied strain (ε). OFF-state
resistance is ~ 1012 Ω.
0.0
2.0x10
6
4.0x10
6
6.0x10
6
Dose (Rad)
Fig. 7. Evolution of resistance prior to exposure, and after exposure to
630 krad(Ge20Se80), 2.61 Mrad(Ge20Se80), 3.29 Mrad(Ge20Se80), 3.83
Mrad(Ge20Se80), 4.49 Mrad(Ge20Se80) and 5.21 Mrad(Ge20Se80)
A similar decrease of the resistance has been observed on
devices exposed to UV light. For those UV exposed devices,
prior to exposure, the device is in a high resistance OFF state
of around 1012 Ω, since the undoped ChG (no Ag in film) acts
as a dielectric layer between the electrodes. After 19.22 J/cm2
of UV exposure, the Ag has diffused laterally by more than
0.5 mm between the electrodes. A complete “saturation” of the
ChG film with Ag is observed at 43.25 J/cm2 of UV exposure.
As mentioned previously, the simple room temperature
fabrication method developed in this study and the flexibility
of the Ge20Se80 based glass enables the manufacture of flexible
sensor structures. However, the performance of the sensor
under bending stresses should also be assessed to ensure that
the materials do not undergo major electrical changes as a
result of stress. Fig. 8 shows the change in ON-state resistance
when the exposed device is subjected to parallel and
perpendicular stress, both tensile and compressive. For all
measurements, several sensor samples were bent for 96 h on
cylindrical surfaces of various radius and then probed on a flat
surface to measure their resistance. As shown in Fig. 8, the
ON-state resistance of the device did not show significant
degradation and falls within the range of resistance values of
Fig. 9: ON-state resistance as a function of temperature annealing T (OC).
OFF-state resistance is ~ 1012 Ω
Finally, the effect of elevated operating temperature on
sensor characteristics was assessed. Fig. 9 shows the ON-state
resistance as a function of temperatures between room
temperature and 150 °C. Once again, the ON-state resistance
falls within the range of resistance values of the sensor devices
measured before temperature stressing (10 – 12 kΩ).
Although the OFF-state resistance shows no change with
temperature, it is understood that prolonged high-temperature
annealing could cause silver diffusion in the ChG [25].
the simulation accurate was the diffusion coefficient of the
silver ions in Ge20Se80 chalcogenide glass. As it is commonly
known, Ag does not have a constant diffusion coefficient.
Instead, this value is dependent on the silver concentration
within the chalcogenide glass. From ref. [26], the diffusion
coefficient was determined to be 1.15 x10-11 m2/sec for a Ag
concentration of 10 at.% Ag and 1.16 x10 -10 m2/sec for 20
at.% Ag. An approximation was made of source Ag
concentration at the interface between the silver and the
chalcogenide glass film. This is an arbitrary number of atoms
that will not affect the overall simulation result. Initial
concentration was assumed to be 1000 mol/m3.
The diffusion simulations were performed by applying
adjusted Fick’s diffusion laws and calculating the diffusion
dependence on time/concentration using COMSOL
simulations. The continuity equation for this process is
A. Finite Element Simulation Results
Fig. 10. COMSOL simulations modeling the diffusion profile achieved using
UV light which is similar to the 60Co gamma irradiation induced Ag lateral
diffusion shown in Fig. 4.
1.2
0 hr
2.5 hr
3.5 hr
6.5 hr
Normalized Concentration
1.0
0.8
0.6
0.4
0.2
where c is Ag concentration, i is the value from the previous
iteration, D is the diffusion coefficient, and v is the velocity of
the diffusant.
After creating the specific device structure, initial
conditions were assumed such that the perimeter of the
simulation area was always at 0 mol/m3 and all the diffusing
species are confined within this area. The source of the
diffusing species was created such that a constant
concentration of 1000 mol/m3 is always present around each
of the silver sources independent of the previous iteration
result. Due to the velocity and diffusion coefficient values
chosen, the simulation time was similar to the experimental
time. The entire transient simulation was performed from t=0
to t=23,400 seconds with 60 time steps. The results in Fig. 10
are given for the various time intervals.
Quantitative simulations were performed where Ag
concentrations were extracted along cut lines made diagonally
between two Ag electrodes. Fig. 11 plots the evolution of the
normalized neutral Ag concentration along the cut lines. The
figure shows that with increasing dose, Ag diffuses into the
ChG film. Increasing radiation dose causes the ChG film to
contain higher quantities of Ag, thereby lowering the
resistivity of the ChG film between the Ag electrodes.
V. CONCLUSION
0.0
-0.8
-0.6
-0.4
-0.2
0.0
0.2
0.4
0.6
0.8
Distance (mm)
Fig. 11. Evolution of Ag distribution between electrodes with increasing
UV exposure time.
The lateral diffusion of Ag into the ChG was modeled using
COMSOL. Fig. 10 shows simulation results that broadly
match the experimental results of the devices exposed to UV
light. Based on the results obtained after 2.5 hrs of exposure,
the diffusion distance is approximately 0.5 mm from each of
the electrodes. This value was used to determine the rate of
diffusion as 5.6 x 10-8 m/s. Another quantity required to make
In this work, we investigated flexible radiation-sensing
devices which rely upon resistance change in flexible Ge20Se80
films as a result of radiation-induced diffusion of Ag into the
chalcogenide glass. The sensors were stressed mechanically
by bending them inward and outward to produce a range of
compressive and tensile stresses and were also exposed to
elevated temperatures to assess their stability. The OFF- and
ON-state resistances of the devices did not show significant
degradation following mechanical or thermal stress although
further research is needed to understand the limits of such
operational trauma. COMSOL simulations of Ag transport
reproduced the experimental results by modeling ion transport
equations and their reactions with electrons and holes
generated during radiation exposure. The results of this study
are promising as they reveal that very simple, low cost
fabrication techniques can be employed to create a flexible
radiation sensor capable of withstanding high bending strain
without compromising device functionality or structural
integrity. The sensors are thin, lightweight, and capable of
instantaneous readout with a low voltage. All of these positive
factors make this technology a strong candidate in the field of
electronic radiation dosimetry.
ACKNOWLEDGMENT
The authors would like to thank Dr. James Reed of DTRA
for his support of this work. Also, we gratefully acknowledge
the use of facilities within the LeRoy Eyring Center for Solid
State Science (LE-CSSS) and Center for Solid State Electronic
Research (CSSER) at Arizona State University.
[18]
[19]
[20]
[21]
[22]
[23]
[24]
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